Radiotherapy devices are for example known from GB 2471750 A. These typically include a treatment table, a support device, for example in the form of a rotating gantry, and a therapeutic radiation source fixed to the support device. During the specified normal operation of the radiotherapy device, the therapeutic radiation source generates ionizing radiation, such as used for the X-radiation or gamma radiation, or high-energy electrons, protons or ion beams.
For example, to check the radiotherapy device or to schedule an exposure of a patient by means of the radiotherapy device, water phantoms can be used that are known for example from U.S. Pat. No. 7,420,160 B2.